The present invention relates to processes for producing potassium sulfate, and more particularly, to processes for producing potassium sulfate, sodium sulfate and sodium chloride from potash and a source of sodium sulfate.
Sodium Sulfate Production PA1 Potassium Sulfate Production
Various processes are known for producing sodium sulfate from hydrated sources of sodium sulfate. High-quality commercial grades of sodium sulfate are usually produced from Glauber's salt (Na.sub.2 SO.sub.4 *10H.sub.2 O). Glauber's salt is obtained from natural deposits ("mirabilite") existing in various cold climates. Glauber's salt is also produced by cooling a natural brine, a solution obtained by solution-mining, or a process stream. The cooling is effected in ponds or in crystallizers (surface-cooled or vacuum-cooled).
Anhydrous sodium sulfate is typically produced from Glauber's salt by evaporative crystallization in a multiple-effect or mechanical vapor recompression (MVR) evaporator, by dehydration in a rotary dryer, or by melting followed either by evaporation or by salting out with sodium chloride. The melting of Glauber's salt to precipitate anhydrous sodium sulfate generally produces an unacceptably fine product material. Moreover, Glauber's salt often contains insoluble matter which is unacceptable in high grade anhydrous sodium sulfate. Hence, dissolution, filtration (and auxiliary separation methods such as desliming), and evaporative crystallization operations are necessary to obtain material of the proper quality. Generally, some mother liquor is purged in order to keep impurities from precipitating out with the product. Alternatively, the Glauber's salt can be melted to produce low-quality "salt-cake" grade sodium sulfate. The saturated mother liquor is then filtered and evaporated to produce high-grade sodium sulfate.
In the production of potassium sulfate from potash and sodium sulfate, thermodynamic and economic constraints dictate that the potassium sulfate be produced in two stages. In conventional processes, these stages consist of:
1) Production of glaserite (K.sub.3 Na(SO.sub.4).sub.2) from sodium sulfate, potash, and Stage 2 liquor;
2) Production of potassium sulfate from potash, water, and glaserite from Stage 1;
The mother liquor produced in Stage 1 contains substantial quantities of dissolved potassium and sulfate, which generally warrants a recovery operation. The currently-known processes differ primarily in the scheme used to retrieve these potassium and sulfate values.
Several processes (hereinafter "Type I" processes) take advantage of the different solubility behaviors of potassium chloride, sodium chloride, and sodium sulfate/Glauber's salt at high and low temperatures. The effluent from Stage 1, of composition `a` (at 25.degree. C.) (see FIG. 1b), is cooled to about 0.degree. C., precipitating Glauber's salt for reuse and possibly some sodium chloride, depending on the water balance in the system. The potassium values are concentrated in the aqueous phase.
After separation, the solution is evaporated at high temperature, yielding sodium chloride and further concentrating the potassium ions in solution. The sodium chloride is removed as the process by-product, and the hot liquor is cooled, precipitating potassium as KCl and/or glaserite, which is subsequently returned to the reaction stages. Alternatively, the hot brine is reacted with Glauber's salt recovered from the cooling crystallization stage to produce a glaserite suspension, which is returned to Stage 1.
Other cyclic processes (hereinafter "Type II" processes) take advantage of the different solubility behaviors of potassium chloride and sodium chloride at high temperatures. The quantity of water added to the reaction stages is set such that glaserite and solution `b` (at 25.degree. C.) are produced (FIG. 1b). The glaserite is then reacted with potash and water to produce the potassium sulfate product and a liquor of composition `c` (at 25.degree. C.). The liquor is returned to Stage 1. The effluent liquor from Stage 1 is evaporated at high temperatures (75.degree.-110.degree. C.), producing pure NaCl, and the end liquor is returned to Stage 1.
It must be emphasized that the production of potassium sulfate from potash and sodium sulfate is a low value-added process, even when the sodium chloride by-product can be marketed. The multi-stage processes described above are both capital-intensive and energy-intensive.
The Type I processes are particularly complex, requiring a large number of unit operations. These include 4 to 6 filtration steps, not including filtration of the washed potassium sulfate product. Moreover, cooling crystallization is used to bring the temperature of the Stage 1 effluent to 0.degree. C. The heat of crystallization of Glauber's salt, which is substantial (18.4 kcal/m), must also be removed at low temperatures. The cooling and heating costs associated with this stage, coupled with expensive equipment (such as crystallizers, heat exchangers, coolant system, and the like) are a serious disadvantage.
The Type II processes have no cooling stage below ambient conditions. However, the recycle stream is much larger (.about.10 tons per ton K.sub.2 SO.sub.4 produced), which increases energy consumption. The low ratio of water evaporated to throughput in the evaporative crystallization stage drastically reduces the natural slurry density, requiring larger crystallizers and/or more sophisticated crystallization technology.
Although Glauber's salt is a relatively inexpensive source of sodium sulfate, the additional water from the Glauber's salt decreases the conversion in the reaction stages and increases the sulfate composition of the Stage 1 effluent. Some cyclic processes cannot be operated using Glauber's salt while others require additional unit operations (for example, evaporation).
To date, there is no economically viable industrial process for producing agricultural-grade potassium sulfate from sodium sulfate or Glauber's salt.
Thus, there is a widely recognized need for, and it would be highly advantageous to have, a process for producing potassium sulfate from sodium sulfate which would be more efficient and more economical that heretofore known.